1. Field of the Invention
The present invention relates to an amorphous photovoltaic element. More particularly, the present invention pertains to an amorphous photovoltaic element such as a solar battery including a pin type junction having excellent electric properties, a high photoelectric conversion efficiency, and a high reliability.
2. Description of the Prior Art
A solar battery is a typical example of an electronic device utilizing the photovoltaic effect. The solar battery permits the conversion of solar energy, which has a quite wide spectral distribution spreading over the range of from 0.3 to 3 .mu.m, or the light energy of other light sources into electric energy and therefore, it serves as a clean, inexhaustable future energy source. In the solar battery, the conversion of light energy to electric energy is performed due to the photoelectric conversion effect which is an essential property of different kind of junctions such as heterojunctions, pn or pin (or nip) junctions of a semiconductor, or Schottky junctions. These junctions absorb incident light and form pairs of electrons at the boundary thereof and (positive) holes which can be outputted and used as an electric current, for example, in the case of the solar battery.
Recently, amorphous semiconductor films such as an amorphous silicon (a-Si:H) film have been suggested as a material for preparing photovoltaic elements such as solar batteries, because of their properties such that an amorphous layer having a low thickness and a relatively large area may be prepared. Layers having different compositions may easily be realized, and the electrical and optical properties of the layer may be widely changed or controlled. For instance, solar batteries obtained using amorphous semiconductor materials have an absorption coefficient with respect to light of about 500 nm, which is the peak value in the solar energy distribution, ten times larger than that of crystalline silicon. These amorphous layers may be formed at a relatively low temperature condition and may be directly formed from starting materials by, for example, the glow discharge discharge technique. Moreover, the junctions such as mentioned above can also be easily formed.
In order to design, produce, and practically use a photovoltaic element, in particular, a solar battery, it is quite important to enhance or improve the photoelectric conversion efficiency thereof. Under such circumstances, Y. Hamakawa et al. (Appl. Phys. Lett., 1979, 35, 187) proposes a solar battery composed of a multi-layer structure such as the pin/pin/ . . . /pin or nip/nip/ . . . /nip type structure in which an asymmetric electromotive force is generated due to the recombination taking place at the boundary between the n-type and p-type layers directly in contact with each other. In this kind of solar battery according to this article, a number of unit solar cells are electrically communicated in series and accordingly it is expected to supply a high electromotive force.
In addition, it is desirable to strictly select the starting material for manufacturing solar batteries and a structure which makes it possible to impart a good absorption coefficient, with respect to the entire solar energy distribution extending over a wide spectral range, to a solar battery in order to substantially improve the conversion efficiency thereof. For that purpose, it has been proposed that a certain element (a modifier) be added to an a-Si:H layer to change the width of the forbidden band thereof. For such a modifier, Group IV elements of the Periodic Table capable of forming 4-cordination bonds with silicon may be utilized.
If carbon, for instance, is used as the modifier, the width of the forbidden band of the amorphous silicon layer becomes wider than that of a layer free of the modifier. Therefore, if the layer of amorphous silicon containing carbon atoms as a modifier is used as the p-type layer situated at the side which is irradiated with the incident light, a solar battery which effectively converts the energy of the light having relatively short wave length to electric energy is provided, that is, the use of such modifier permits an improvement in the photoelectric conversion efficiency (see, for example, Y. Hamakawa et al., Appl. Phys. Lett., 1981, 39, 273). However, if germanium (Ge) (for example, amorphous silicon germanium=a-SiGe:H, see, G. Nakamura et al., J. Appl. Phys., 1981, 20, Suppl. 20-2, 227), tin (Sn), lead (Pb) or the like is used as the modifier, the forbidden band of the amorphous silicon layer becomes narrower than that of a layer free of the modifier. Therefore, if such modified amorphous silicon layer is used to form an i-type layer, a solar battery including such i-type layer is improved in the sensitivity to light of longer wavelengths and thus effectively absorbs light having a long wavelength. In general, it becomes possible to effectively utilize the solar energy distributed over a wide spectral range by arranging the layers constituting a solar battery so that the width of the forbidden band of the layers is gradually narrowed from the side irradiated with the incident light by the use of p-type and i-type layers having properties improved by the addition of modifiers.
An amorphous silicon solar battery comprising an i-type layer composed of an a-Si:H film which provides a high photoelectric conversion efficiency and which is a cheap thin film forming material for this type of solar battery, is expected to have excellent properties because of its high sensitivity to light of long wavelength. Thus, a solar battery comprised of a glass substrate/a transparent electrode/a pin junction/a metal electrode, in which the p-type and n-type layers are comprised of an a-Si:H film and the i-type layer is composed of an a-SiGe:H film may be manufactured. However, the pin type of solar battery having the construction mentioned above is inferior in its battery properties. Therefore, it is required to improve the properties thereof in order to put solar batteries having such structure into practical use.
Referring to the conventional type of solar battery mentioned above in which a p-type layer of a-Si:H is disposed adjacent to the i-type layer of a-SiGe:H, impurities such as boron (B) are added to the p-type layer to assure the p-type nature thereof, since the a-Si:H layer has a weak n-type nature. However, the impurities tend to penetrate into the neighbouring i-type layer due to diffusion or they contaminate the i-type layer during the deposition procedures (in the film forming chamber). Thus, the i-type layer may be converted to a p-type one, which may cause a lowering of the strength of the internal field and reduction in the output properties. Moreover, lattice mismatching is often observed at the boundary between the i-type layer to which germanium (Ge) is added and the p-type layer free of Ge (hereafter referred to as the p/i boundary). These defects result in an extreme reduction of the output current and the fill factor (FF) as well as other output properties of the solar battery. That is, the output current, the open circuit voltage (Voc) and the factor (FF) of the solar battery are all reduced.
In general, the p-type layer serves to conduct the light as much as possible to the neighbouring i-type layer and also acts as an electrode of the battery. However, the added element (Ge) in the i-type layer penetrates into the p-type layer due to diffusion during the formation of the former, in particular, around the p/i boundary, which leads to a reduction of the bandgap energy of the p-type layer. Thus, the film characteristics inherent to the p-type layer are remarkably impaired and the latter does not perform the role of transmitting light to the neighbouring i-type layer. As a result, the amount of light which arrives at the i-type layer is significantly reduced and in turn this results in a reduction in the output current of the solar battery. Moreover, the reduction in the bandgap energy causes a lowering of the built-in potential and accordingly the open circuit voltage, having a close relationship to the built-in potential, is lowered.
These phenomena i.e., the penetration of boron (B) included in the p-type layer into the i-type layer due to diffusion, the electric junction loss due to the lattice mismatching at the p/i boundary, and the diffusion of Ge atoms contained in the i-type layer into the p-type layer mentioned above take place not only during the film formation procedures but during the use of the devices having the structure described above, such as solar batteries, and they cause a reduction in the output properties thereof.